In a report, a line refers to the column headings that identify the contents of each column. A report must have at least 1 line. However, you can create divide a report section into several "lines" to help you organize your report data in a meaningful way.
In this example, the report line is highlighted in red.
If you looked at the Columns tab in the report settings, it would look like this...
See how each column is assigned to Line 1 on the Columns tab. Now, look at the report printout and you can see how each column assigned to Line 1 also appears as a column heading in the report section.
How do I add/remove a report line?
How do I view a different report line?
How do I assign a column to a different report line?